The present invention relates generally to improvements in liquid coating techniques, and more particularly to an improved apparatus and method for applying a controlled amount of a liquid to a substrate such as a printed circuit board. The invention has particular utility in the application of a fluid solder flux to a printed circuit board which is to be mass soldered and will be described in connection with such utility, although other utilities are contemplated.
Commercially available mass soldering systems include a conveyor system for conveying a printed circuit board along a path through various work stations. The printed circuit board has components mounted thereon with leads positioned adjacent lands to which the leads are to be soldered.
In a typical mass soldering system, a liquid solder flux is applied to a solder board prior to soldering. The board is then preheated to activate the flux, and the preheated board is then passed through the mass soldering station. Preheating also serves to evaporate flux solvent, and reduce thermal shock. The flux serves both as a cleaning and wetting agent to enhance the quality of the solder joint formed between the component leads and circuit board lands.
Several flux application methods have been used historically in the industry. One method pumps flux through a nozzle to form a standing of wave of flux fluid through which the bottom surface of the board is passed. Another popular method is to aerate the fluid flux to produce bubbles which rise up to wet the bottom surface of the board. However, both of these methods do not allow good control over the amount of flux deposited on the board. Another method and one which can be used to apply very small amounts of flux to the bottom side of the board is spraying. One method of spraying flux is to rotate a mesh drum through a bath of flux and then blow air through the mesh to create a mist of flux. While such prior art fluxing systems have achieved commercial success, such systems suffer from disadvantage of evaporation of flux solvent during use. Evaporation of the flux solvent results in a progressive change in specific gravity of the flux. As time goes on, the relative solid content of the flux increases with the result that more solids may be applied to and remain on the board. Generally, the preheaters are set to flash off the flux solvent and preheat the printed circuit board to activate the flux and reduce thermal shock. However, as the flux deposit increases, the deposited flux takes more of the heat, leaving less for activating flux and less for reducing thermal shock. This results in variation in the soldering process and potentially poor solder joints. Another result of uncontrolled solvent evaporation is increased flux consumption because dragout on the boards is greater. Excessive flux consumption means increased flux and cleaning costs. While solvent evaporation can be compensated for by periodically adding solvent, variation in flux concentration remains a problem.
More recently several manufacturers have introduced closed system spray fluxing systems employing either a single spray nozzle on a moveable arm, or a plurality of fixed spray nozzles. Since such commercially available spray fluxing systems are closed systems and thus solve the problem of solvent evaporation and resulting density variation, flux application uniformly has not proven to be entirely satisfactory. Moreover, single nozzle systems are relatively mechanically complicated. Also, such commercially available closed system spray fluxing systems employ pressurized vessels. This presents a potential safety hazard, and many local fire safety codes may not permit flammable fluids to be stored in a reservoir under pressure.
It is accordingly an object of the present invention to overcome the aforesaid and other problems of the prior art.
Another object of the present invention is to provide a novel spray fluxing system, i.e. apparatus and method, capable of accurately and uniformly depositing flux onto a printed circuit board.